Rotatable assembly for dynamoelectric machines having means for reducing release of magnet material particles therefrom

ABSTRACT

A magnet assembly having means for reducing release of magnet material particles from the magnet element includes a body of permanently magnetizable material and a shell at least partially surrounding the body. The shell is disposed and located so as to reduce release of particles shed from the surface portion of the body which is surrounded by the shell, and is preferably attached to the body by an adhesive which bonds the adjacent surfaces of the shell and the body together. A plurality of such magnet assemblies may be fastened to a flux ring in order to provide a rotatable assembly for a dynamoelectric machine in which migration of magnet material particles into the stator-rotor air gap is reduced. In accordance with a preferred embodiment, the magnet assemblies of the present invention are employed in a molded rotor assembly for such a machine.

BACKGROUND OF THE INVENTION

This invention relates to rotatable assemblies for dynamoelectricmachines, such as electronically commutated motors. More particularly,it relates to a means for reducing shedding of particles from thepermanently magnetizable materials which are typically employed to formthe magnet elements for such machines.

In conventional designs of rotatable assemblies for dynamoelectricmachines, such as, for example, electronically commutated motors,permanent magnet elements are fastened to the outer surface of a steeldrum which is attached to a rotatable shaft. In such designs, thepermanent magnet elements form the outer surface of the rotor assembly.The permanent magnet elements typically used in electronicallycommutated motor rotors are formed from permanently magnetizablematerial which has been made by powder metallurgical techniques. Thesematerials are brittle and have a tendency to release microscopicparticles or chips from their surfaces, especially during rotation ofthe rotor assembly. Some of these released particles may migrate throughthe stator-rotor air gap and settle on the inner surface of the stator,thereby affecting the stator-rotor gap and degrading motor performance.

Various schemes have been employed to retain magnet material elements,such as permanent magnets or the like, against displacement from theirproper positions in a rotatable assembly for dynamoelectric machines. Itwould appear that some of these schemes would also serve, at least inpart, to reduce migration of magnet material particles into thestator-rotor gap. One such scheme is disclosed in U.S. Pat. Nos.4,242,610, issued Dec. 30, 1980 to F. B. McCarty et al., and 3,531,670,issued Sept. 29, 1970 to D. C. Loudon. Those patents appear to describeheat shrinking a metal or metal alloy sleeve or band about the rotorcore into displacement-preventing engagement with a set of magnetmaterial elements arranged or otherwise seated in assembly positionsabout the circumference of the rotor core. While such an arrangementappears to at least partially reduce migration of magnet materialparticles from the outer surfaces of the magnet elements, this prior artarrangement requires undesirable heating during assembly, is too bulkyand cumbersome for some applications, and is relatively expensive. Toprovide the metallic bands disclosed in those patents with adequatestrength to hold the magnet elements in position, a relatively thickmaterial must be used. Furthermore, even if the magnet elements arefastened to the rotor core by a supplementary means such as adhesivebonding, it is still difficult to provide such bands with adequatestrength using a thin material. In addition to the hoop stresses in themetallic band generated during rotation of the rotor assembly bycentrifugal forces acting on the band's own mass, radial expansion ofthe rotor assembly during rotation also imposes stresses on the band.The amount of radial expansion of the rotor assembly is expected to begreater than that of the metallic band for several reasons. First,during rotation the flux ring exhibits an incremental expansion due tocentrifugal loading by the attached magnet elements. Also, if the magnetelements are adhesively bonded to the flux ring, the magnet elementstend to move radially at a different expansion rate than that of themetallic band, due to the low elastic modulus of the adhesive.Additionally, if the rotor core is made of plastic, the rotor core alsotends to radially expand at a different rate than the metallic band,because of the core's lower elastic modulus. To withstand the stressesimposed by these mechanisms, the metallic bands disclosed by U.S. Pat.Nos. 4,242,610 and 3,531,670 appear to require relatively highmechanical strength. U.S. Pat. No. 3,221,194, issued Nov. 30, 1965 to A.B. Blackburn, also appears to disclose a scheme which would tend toreduce migration of magnet material particles into the stator-rotor gap.That patent describes dipping a rotor core with permanent magnetmaterial elements arranged thereon into a plastic bath. When cured, theplastic forms an encapsulating layer over both the rotor core and themagnet elements, securing them together. However, for similar reasons asdiscussed above, such a plastic layer would also be subjected to bothhoop stresses and stresses imposed by radial expansion of the rotorassembly during rotation.

Accordingly, it is an object of the present invention to provide amagnet assembly having means for reducing release of magnet materialparticles from the surface of the magnet element.

It is a further object of the present invention to provide a rotatablemagnet assembly in which the stresses imposed on the means employed forreducing release of magnet material particles from the magnet elements,caused by radial expansion of the assembly during rotation, are reduced.

It is another object of the present invention to provide a rotatableassembly for dynamoelectric machines in which migration of magnetmaterial particles into the stator-rotor gap is reduced.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a magnetassembly having means for reducing release of magnet material particlestherefrom comprises a body of permanently magnetizable material and ashell at least partially surrounding the body. The shell is disposed andlocated so as to reduce release of particles shed from the surfaceportion of the body which is surrounded by the shell, and is preferablyattached to the body by an adhesive which bonds the adjacent surfaces ofthe shell and the body together.

In accordance with another aspect of the present invention, a pluralityof such magnet assemblies are fastened to a flux ring in order toprovide a rotatable assembly for a dynamoelectric machine. The magnetassemblies are disposed about the outer circumference of the flux ringso that they are situated adjacent to each other and so that the outersurfaces of the magnet assemblies form the curved surface of a cylinder.The rotatable assembly may also include an axial shaft extending alongthe central axis of the flux ring, and a matrix molded between the shaftand the inner circumference of the flux ring.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention itself, however, both as to itsorganization and its method of practice, together with further objectsand advantages thereof, may best be understood by reference to thefollowing description taken in conjunction with the accompanyingdrawings, in which:

FIG. 1 is a perspective view schematically illustrating one embodimentof a magnet assembly in accordance with the present invention;

FIGS. 2-4 are cross-sectional end views schematically illustratingalternative embodiments to the embodiment shown in FIG. 1;

FIG. 5 is a cross-sectional end view schematically illustrating aportion of one embodiment of a rotatable assembly for a dynamoelectricmachine in accordance with the present invention;

FIG. 6 is a perspective view schematically illustrating a portion ofanother embodiment of a rotatable assembly for a dynamoelectric machinein accordance with this invention; and

FIG. 7 is a cross-sectional end view schematically illustrating a moldedrotor assembly in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 schematically illustrates one embodiment of a magnet assemblyhaving means for reducing release of magnet material particlestherefrom, in accordance with the present invention. A body ofpermanently magnetizable material 10 is at least partially surrounded byshell 12, with shell 12 being disposed and located so as to reducerelease of particles shed from the surface of body 10 which issurrounded by shell 12. Attaching means 14 serves to attach shell 12 tobody 10. As shown in the cross-sectional views of FIGS. 2-4, attachingmeans 14 preferably comprises an adhesive which bonds the adjacentsurfaces of body 10 and shell 12 together. Cladding the surface of body10 by means of shell 12 and adhesive attaching means 14, in the mannerillustrated in FIG. 1, results in containment of microscopic particlesand chips that would otherwise be released from the surface of body 10.In order to minimize the effect of shell 12 on the magnetic performanceof body 10, shell 12 preferably comprises a thin sheath of materialwhich does not significantly affect the magnetic properties of body 10.In one embodiment, shell 12 comprises stainless steel foil.

In the embodiment shown in FIG. 1, shell 12 is located adjacent to thetop surface and to both side surfaces of body 10, so as to reducerelease of particles shed from those three surfaces of body 10. FIGS.2-4 are cross-sectional end views schematically illustrating alternativeembodiments to that shown in FIG. 1. In the embodiment of FIG. 2, shell12 is located adjacent to only the top surface of body 10, so as toreduce release of particles shed from that surface. In the embodimentsof FIGS. 3 and 4, shell 12 is located adjacent to the top surface, bothside surfaces, and at least a portion of the bottom surface of body 10,so as to reduce release of particles shed from the top surface, fromboth side surfaces, and from at least a portion of the bottom surface ofbody 10. If the magnet assemblies illustrated in FIGS. 1-4 are to bemounted on a flux ring, with the bottom surface of body 10 attached tothe outer surface of the flux ring, the embodiments shown in FIGS. 3 and4 would likely result in a loss of magnetic flux between body 10 ofpermanently magnetizable material and the flux ring. Because, in theembodiment shown in FIG. 3, shell 12 only partially covers the bottomsurface of body 10, the loss of magnetic flux would be smaller for thatembodiment than for the embodiment shown in FIG. 4. However, for someapplications, the embodiment of FIG. 4 may be preferred over that ofFIG. 3, since the embodiment of FIG. 4 results in a more uniformmagnetic flux pattern. As between the magnet assemblies of FIGS. 1 and2, the embodiment shown in FIG. 1 provides a smoother overall outersurface for the magnet assembly, and is preferred for applications suchas rotors for dynamoelectric machines.

In each of the embodiments illustrated in FIGS. 1-4, body 10 ofpermanently magnetizable material has been shown as a generallybar-shaped body having generally arcuately shaped top and bottomsurfaces. Although the principles of this invention apply to virtuallyany body shape, the bar-shaped bodies shown are especially useful forsuch applications as electronically commutated motors. FIG. 5schematically illustrates a portion of one embodiment of a rotatableassembly for such a dynamoelectric machine, in accordance with thepresent invention. The rotatable assembly comprises flux ring 16, forproviding a medium of distribution for magnetic flux, and a plurality ofmagnet assemblies disposed about the outer circumference of flux ring16. In the embodiment shown in FIG. 5, the magnet assemblies arefastened to flux ring 16 by adhesive material 18. Each magnet assemblyis of the type shown in FIG. 1, and includes generally bar-shaped body10 of permanently magnetizable material, shell 12 at least partiallysurrounding body 10, and attaching means 14 for attaching shell 12 tobody 10. For each magnet assembly, shell 12 is disposed and located soas to reduce release of particles shed from the surface of body 10 whichis surrounded by shell 12. The magnet assemblies are fastened to fluxring 16 so that they are situated adjacent to each other and so that theouter surfaces of the magnet assemblies form the curved surface of acylinder. With the outer surface and both side surfaces of each body 10of permanently magnetizable material being surrounded by shell 12, andwith the inner surface of each body 10 being attached to flux ring 16,in the manner shown in FIG. 5, the amount of magnet material particlesreleased from the magnet material elements during rotation of theassembly is significantly reduced. Hence, the rotatable assembly of thepresent invention may be employed to advantage in a dynamoelectricmachine in order to reduce migration of magnet material particles intoand through the stator-rotor air gap. At the same time, the meansemployed for reducing release of magnet material particles does notrequire the use of materials having relatively high mechanical strength.In the rotatable assembly of the present invention, shell 12 is notsubjected to stresses imposed by radial expansion of the assembly duringrotation. Furthermore, since shell 12 may comprise a relatively thinmaterial, the hoop stresses generated during rotation of the assembly bycentrifugal forces acting on the mass of shell 12 are relatively small.Accordingly, the means for reducing release of magnet material particlesemployed by the present invention may comprise lightweight, relativelylow strength materials.

In the embodiment of FIG. 5, the magnet assemblies are shown as being ofthe type illustrated in FIG. 1, with shell 12 of each magnet assemblybeing located adjacent to the outer surface and to both lateral surfacesof body 10 so as to reduce release of particles shed from thosesurfaces. However, the magnet assemblies illustrated in FIGS. 2-4 mayalso be used. As was noted above, shell 12 preferably comprises amaterial which does not significantly affect the magnetic performance ofbody 10 of permanently magnetizable material, such as, for example,stainless steel foil. Attaching means 14 preferably comprises anadhesive which bonds the adjacent surfaces of shell 12 and body 10together.

In one embodiment of the rotatable assembly of the present invention,the flux ring employed comprises a unitary annular structure,illustrated in FIG. 5 as flux ring 16. FIG. 6 schematically illustratesa portion of an alternative embodiment in which the flux ring comprisesa plurality of flux ring segments 20, with each flux ring segment 20having mounted thereon one or more of the magnet assemblies shown inFIGS. 1-4. In the specific embodiment shown in FIG. 6, the magnetassembly of FIG. 1 is employed. As shown by the cross-sectional end viewof FIG. 7, which schematically illustrates a molded rotor assembly inaccordance with the present invention, in order to form a rotatableassembly, flux ring segments 20 are circumferentially arranged in aspaced-apart relationship about a central axis and held in position by asupporting means so that flux ring segments 20 form the general shape ofan annulus. In the embodiment of FIG. 7, the supporting means comprisesmolded matrix 26. The rotatable assembly embodied by FIG. 7 may beformed, for example, by fastening one or more magnet assemblies to eachflux ring segment 20, arranging the flux ring segments having the magnetassemblies fastened thereto in an appropriately shaped mold, and thenmolding in a matrix material, such as, for example, phenolic.

The means for fastening the plurality of magnet assemblies to eitherflux ring 16, shown in FIG. 5, or flux ring segments 20, shown in FIGS.6 and 7, may comprise adhesive material 18 which bonds the magnetassemblies to either flux ring 16 or to flux ring segments 20,respectively. As illustrated in FIG. 6, the fastening means may alsocomprise a plurality of locking tabs 22, which may be formed as anintegral part of either flux ring 16 shown in FIG. 5 or flux ringsegments 20 shown in FIG. 6. Locking tabs 22 are disposed and located sothat they are in gripping engagement with confronting parts of themagnet assemblies, so as to at least in part retain the magnetassemblies against displacement from their respective positions. Also,while the means for attaching matrix 26 to either flux ring 16 or fluxring segments 20 may comprise adhesive bonding, the attaching means mayalso comprise protuberances which are formed on either flux ring 16 orflux ring segments 20 and which are embedded in matrix 26. Asillustrated in FIG. 6, in one embodiment the protuberances compriseanchoring tabs 28 which are formed as an integral part of either fluxring 16 or flux ring segments 20. Anchoring tabs 28 extend radiallyinwardly beyond the inner surface of either flux ring 16 or flux ringsegments 20, for anchoring engagement in matrix 26. In the embodimentshown in FIG. 6, locking tabs 22 and anchoring tabs 28 are located atthe axial ends of flux ring segments 20. However, other locations may beused, such as, for example, along the lateral surfaces of segments 20,or between the lateral surfaces of segments 20 in locations intermediatethe axial ends of the segments.

In yet another embodiment of the present invention, the rotatableassembly described above is employed to form a molded rotor assembly fora dynamoelectric machine. As illustrated in FIG. 7, such a molded rotorassembly includes one of the embodiments of a rotatable assembly inaccordance with the present invention, and further comprises axial shaft24 and molded matrix 26. Shaft 24 extends along the central axis of theannularly shaped flux ring formed by either flux ring 16 shown in FIG. 5or flux ring segments 20 shown in FIGS. 6 and 7. Matrix 26 is moldedbetween shaft 24 and the inner circumference of the flux ring so thatmatrix 26 is at least in part in supporting relationship between shaft24 and either flux ring 16 or flux ring segments 20. Each body 10 ofpermanently magnetizable material is at least partially surrounded byshell 12, in the manner shown in FIGS. 1-4. Regardless of which of theembodiments shown in FIGS. 1-4 is used, shell 12 covers at least theouter surface of each body 10, and the inner surface of each body 10 isattached to either flux ring 16 or flux ring segments 20. Hence, magnetmaterial particles are constrained from being released during rotationof the molded rotor assembly, and the migration of such particles intothe stator-rotor gap is reduced.

The foregoing describes a magnet assembly having means for reducingrelease of magnet material particles from the surface of the magnetelement. The present invention also provides a rotatable magnet assemblyin which the stresses caused by radial expansion of the assembly duringrotation, and imposed on the means employed for reducing release ofmagnet material particles, are reduced. The present invention furtherprovides a rotatable assembly for dynamoelectric machines in whichmigration of magnet material particles into the stator-rotor gap isreduced.

While the invention has been described in detail herein in accord withcertain preferred embodiments thereof, many modifications and changestherein may be effected by those skilled in the art. For example, whilebody 10 of permanently magnetizable material has been shown in theFigures as comprising a ceramic, the principles of the present inventionapply to other materials for which particle shedding is a problem. Also,for the sake of clarity in illustrating the present invention, shell 12is not shown in any of the figures as covering the end surfaces of body10. However, it should be understood that such surfaces could also becovered by shell 12, and that such an embodiment is included in thescope of the present invention. Furthermore, although not shown in theFigures, the flux ring to which the magnet assemblies of the presentinvention are mounted may comprise a steel drum in accordance with theconventional design, in which a plurality of stacked steel laminationsare mounted on the outer surface of a steel shell. Accordingly, it isintended by the appended claims to cover all such modifications andchanges as fall within the true spirit and scope of the invention.

The invention claimed is:
 1. A magnet assembly comprising:a body of permanently magnetizable material including top and bottom surfaces; a shell comprising a thin layer of nonmagnetic material surrounding at least said top surface of only one body for reducing the release of particles therefrom; and means disposed between said body and said shell for attaching said shell to said body; said bottom surface of said body shaped for mounting on a generally cylindrical ring surface; said body shaped such that at least two of said magnet assemblies can be mounted in adjoining relationship on said ring surface so that said top surfaces and said separate shells form a generally cylindrical outer surface overlying said ring surface.
 2. The magnet assembly of claim 1 wherein said shell comprises stainless steel.
 3. The magnet assembly of claim 1 wherein said means for attaching said shell to said body comprises an adhesive which bonds the adjacent surfaces of said shell and said body together.
 4. The magnet assembly of claim 1 wherein said body of permanently magnetizable material comprises a generally bar shaped body further including two side surfaces connecting said top and bottom surfaces.
 5. The magnet assembly of claim 4 wherein said shell surrounds said top surface and said two side surfaces of said body.
 6. A magnet assembly comprising:a body of permanently magnetizable material including top and bottom surfaces, said body of permanently magnetizable material comprising a generally bar-shaped body further including two side surfaces; a shell comprising a thin layer of nonmagnetic material surrounding said top surface said two side surfaces and at least a portion of said bottom surface of said body for reducing the release of particles therefrom; and means disposed between said body and said shell for attaching said shell to said body; said bottom surface of said body shaped for mounting on a generally cylindrical ring surface; said body shaped such that at least two of said magnet assemblies can be mounted in adjoining relationship on said ring surface so that said top surfaces and said shells form a generally cylindrical outer surface overlying said ring surface. 